The present disclosure generally relates to improved ultrasonic waveguides. More particularly, the present disclosure relates to ultrasonic waveguides having improved amplification and reduced modal coupling suitable for use in ultrasonic medical devices such as ultrasonic scalpels, phacoemulsifiers, soft tissue aspirators, other ultrasonic surgical tools, and the like.
Many modern surgical procedures involve the use of ultrasonic surgical devices that typically operate at frequencies between 20 kHz and 60 kHz. These devices have application in many surgical specialties including, for example, neurosurgery, general surgery, and ophthalmic surgery. In general, it is known that ultrasonic surgical devices generate ultrasonic frequency vibratory energy that is applied to an ultrasonic applicator that vibrates longitudinally and which contacts the tissues of a patient. The ultrasonic surgical device may, among other surgical effects, cut, fragment, and/or coagulate the contacted tissues of the patient.
Ultrasonic surgical devices are constrained in their ability to generate ultrasonic frequency vibratory energy due to limits inherent in the physical characteristics of the materials typically used to fabricate the devices. For example, titanium alloys are often used for fabrication of the ultrasonic waveguide that is used to contact the tissues of a patient (i.e., ultrasonic applicator). Titanium alloys have inherent fatigue strength and stress limitations that cannot be exceeded or the ultrasonic applicator will crack and/or break resulting in an unusable tool. As a further example, an ultrasonic waveguide, such as for use as an ultrasonic transducer to convert supplied electrical power to ultrasonic frequency vibratory energy, may be fabricated in a stepped-down fashion; that is, geometrically stepping down the diameter of the transducer. While the smaller diameter end of the transducer will typically have a higher amplitude and thus higher tip velocity due to the stepping down, the stepping down leads to considerable stresses at the step, which can result in less efficient transmission of energy, overheating of the transducer, and increased risk of failure.
Additionally, a phenomenon referred to as “modal coupling” can also be responsible for establishing the upper performance boundary of an ultrasonic surgical device. Modal coupling occurs when the vibratory amplitude of an ultrasonic waveguide of an ultrasonic surgical device is increased to such a level that the ultrasonic frequency vibratory energy at the desired resonant frequency is coupled to other modes of vibration, commonly referred to as “parasitic modes”. The parasitic modes of vibration may be at lower frequencies, near-by frequencies, or higher frequencies, depending on the design of the system. The parasitic modes of vibration may be longitudinal modes or they may be transverse modes, or they may be more complicated coupled modes. Modal coupling is especially troublesome when the ultrasonic waveguide is an elongate probe or catheter with a length greater than one wavelength at the resonant frequency of the particular ultrasonic surgical device; however, modal coupling may also occur for ultrasonic waveguides shorter than one wavelength and for ultrasonic waveguides that are not shaped like an elongate probe, for example, flat or convex radiating surfaces.
The most common type of modal coupling encountered for ultrasonic surgical devices is the stimulation of a lower or near-by frequency transverse mode so that the ultrasonic waveguide vibrates in the desired longitudinal vibratory mode and an undesired transverse vibratory mode simultaneously. This type of coupled vibration can easily cause stresses in the ultrasonic waveguide material sufficient to break the ultrasonic waveguide.
Ultrasonic surgical devices that operate at high vibratory amplitudes may also generate undesirable heat, primarily in the ultrasonic transducer, but also in the material of other ultrasonic waveguides such as in an ultrasonic applicator, due to internal friction and other losses as the ultrasonic applicator vibrates. If the ultrasonic transducer becomes too hot during a typical procedure, active cooling, such as forced air or water cooling, of the ultrasonic transducer is required, making the ultrasonic surgical handpiece more expensive and more cumbersome due to the additional supply lines. Also, if the ultrasonic applicator becomes too hot, unwanted hot spots or unwanted active zones can result, damaging the tissues of a patient.
Based on the foregoing, there is a need in the art for ultrasonic medical devices and ultrasonic waveguides to be used in ultrasonic medical devices that have good amplification but greatly reduced stresses and heat generation. It would also be desirable for the ultrasonic waveguides to have a reduced risk of modal coupling.